A wide variety of cable television system configurations have been devised and implemented. The systems typically include a CATV origination center or headend which transports all of the television channels offered by the cable operator onto a network of coaxial and/or optical cables which are supported in the air above ground by utility poles ("on the strand") or are buried underground. Service to cable television subscribers is usually provided through multiple nodes on the cable network. Each node generally consists of a control box which forwards all of the available channels into one or more subscriber households. The node control box is either attached to a utility pole or, in an underground system, is supported in a pedestal above ground or in a wiring closet in multi-unit dwellings.
Most of the television channels transported from the headend are provided to subscribers who pay a uniform subscription rate. If the subscriber pays the fee, the node associated with his household is programmed to allow all of these "basic" channels to be forwarded into the subscriber's household. However, certain channels, commonly referred to as "premium" channels, are supplied on a per-subscriber basis. That is, if an individual subscriber wishes to receive a particular premium channel, he pays an additional subscription fee for that channel, and access to the channel is enabled for that subscriber. Several approaches to selectively controlling access to premium channels have been implemented.
The most common approach used by cable television systems is to scramble premium channels by altering their video signals such that they cannot be used to produce viewable pictures on a television receiver. This is commonly done by removing or distorting in some controlled fashion the horizontal synchronization signal from the channels' video signals. In such a system, each subscriber is provided with a programmable descrambler or decoder which can be programmed to return the video signal of selected premium channels to normal condition such that the subscriber can watch the selected channel. Thus, if the subscriber wishes to receive a particular premium channel, he contacts his cable operator to request the channel and pays the subscription fee for the channel, and the cable operator programs the subscriber's decoder to decode the selected premium channel.
Typical cable television systems handle up to 82 standard television channels and it is likely that the number of channels will increase up to and beyond 150.
However, most television receivers in use cannot accommodate that large number of channels. To be compatible with television receivers, conventional cable television systems include a converter box installed within the subscriber's household to each television receiver. The converter box is tunable to select one of the channels coming into the household from the network node. In the converter box, the selected channel is up-converted to an intermediate frequency and then down-converted to the carrier signal of a single channel which all television receivers can receive, typically channel 3 or 4, depending upon the area in which the system is operating. With the converter box installed, the subscriber selects a channel by tuning his television receiver to channel 3 or channel 4 and then tuning the converter box to the desired channel. As a matter of convenience, the conversion and descrambling/decoding functions have typically been combined and implemented in a single box, commonly referred to as a converter/decoder unit, located within the subscriber's household.
In an attempt to eliminate the converter box from the subscriber's household, many manufacturers have begun selling "cable-ready" television receivers and video cassette recorders (VCRs) which can be tuned to receive all 82 channels. However, since the decoding function is implemented in the converter/decoder unit, the unit cannot be eliminated if the subscriber wishes to view premium channels. Hence, in these conventional cable television systems, even with a cable-ready television, the television receiver or VCR must be tuned to channel 3 or 4, and the converter box must still be used to select channels. This defeats many of the features available on televisions and VCRs, such as picture-in-picture, recording one channel while viewing another, etc.
Another type of system used to control access to premium channels is known as a "trap" system. In a trap system, each subscriber is provided with multiple traps or filters used to selectively block or enable particular channels. Positive traps are notch filters which remove jamming signals from particular selected premium channels such that the channels can be received by the subscriber. Negative traps are notch filters used to block selected channels. When a subscriber wishes to receive a particular premium channel, a positive trap is implemented at his location, either in a box within his household or in the control box at the node. When a subscriber wishes to prevent a particular channel from being received, a negative trap at that channel is implemented, again, either in the household or at the node.
Trap systems potentially require multiple positive and negative traps for each subscriber, and traps must be manually installed and removed. Another problem with trapping systems is that the bandwidth of notch filters is a function of the center frequency of the filter. The standard cable television channels presently in use have a standard bandwidth of 6 MHZ, regardless of the center frequency of the channel. Since the bandwidth of a notch filter increases with its center frequency, a notch filter at a high frequency channel will have a wider bandwidth than a filter at a lower channel. The higher channel filter bandwidth may overlap channels and therefore filter out channels which should not be blocked.
Another type of system is known as an interdiction system. In an interdiction system, all channels, including premium channels, are transmitted from the headend in the clear. Each of the nodes injects a jamming signal onto all of the premium channel signals to make them impossible to receive. When a subscriber requests a particular premium channel, the node is programmed to remove the interfering signal from the requested channel. Interdiction systems are very hardware intensive since they require special hardware to generate and inject the interfering signal.